Heater for vacuum tube cathode in which the wire of the heater legs has been etched to reduce its diameter

ABSTRACT

An insulatively coated wire-wound reverse helix heater has a coiled body portion and two leg portions. The wire in the leg portions of the heater is etched to reduce its diameter in order to reduce the effect of voltage fluctuations in the heater power supply.

United States Patent [1 1 Larson [111 3,869,692 [451 Mar. 4, 1975 HEATER FOR VACUUM TUBE CATHODE IN WHICH THE WIRE OF THE HEATER LEGS HAS BEEN ETCHED TO REDUCE ITS DIAMETER [75] Inventor: Robert C. Larson, Exeter, NH.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

[22] Filed: May 7, 1973 [21] Appl. No.: 358,016

[52] U.S. Cl 338/296, 338/330, 313/340, 219/552 [51] Int. Cl. H01c 3/00, HOlj 1/22, H05b 3/10 [58] Field of Search 313/333, 337,340; 219/548, 552; 338/330, 333, 217, 218, 296

[56] References Cited UNITED STATES PATENTS 1,081,414 12/1913 Warner 338/333 4/1952 Weyer ..3l3/333 6/1973 Theodosopoulos ..313/337 FOREIGN PATENTS OR APPLICATIONS 476,485 8/1951 Canada 313/337 Primary E.\'aminer-Alfred E. Smith Assistant Examiner-Wm. H. Punter Attorney, Agent, or FirmJames Theodosopoulos [57] ABSTRACT An insulatively coated wire-wound reverse helix heater has a coiled body portion and two leg portions. The wire in the leg portions of the heater is etched to reduce its diameter in order to reduce the effect of voltage fluctuations in the heater power supply.

1 Claim, 3 Drawing Figures PATENTED 5 FIG-.2

FIG.I

PRIOR ART mm F FILAMENT VOLTAGE HEATER FOR VACUUM TUBE CATHODE IN WHICH THE WIRE OF THE HEATER LEGS HAS BEEN ETCHED TO REDUCE ITS DIAMETER BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to insulatively coated wirewound reverse helix heaters generally used in cylindrical cathodes of vacuum tubes.

2. Description Of The Prior Art Reverse helix heaters are generally made of a single length of wire, usually tungsten or tungsten alloy, and have a coiled body portion with two legs longitudinally protruding parallel to each other from one end of the body. The body is insulatively coated, usually with alumina, in order to electrically insulate it from the cylindrical metal cathode in which it is inserted. The ends of the legs are left uncoated in order to permit electrical connection thereof to lead-in wires from a heater power supply. Examples of such heaters are shown in US. Pat. Nos. 3,450,565 and 3,005,926.

SUMMARY OF THE INVENTION An insulatively coated reverse helix heater in accordance with this invention is made of refractory metal wire. Prior to coating, the legs of the heater are etched to reduce the diameter of the wire thereat. In operation, usually only the body of the heater is disposed within a cylindrical cathode, the legs protruding outside the cathode.

When, as sometimes occurs in the case of an unregulated power supply, there is an increase in the heater voltage above its normal operating voltage, the increase in the electrical resistance of the etched portion of the heater is greater than the increase in electrical resistance of the unetched portion. Since most of the heat transfer from the heater to the cathode is conducted and radiated from the part of the heater which is not etched, that is to say, the body, the increase in resistance of the etched portion allows less current flow and thus less power into the unetched portion. This results in less of an increase in cathode temperature than in prior art heaters.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a coated heater in accordance with this invention, part of the heater in section to show the etched portion thereof.

FIG. 2 shows a prior art heater.

FIG. 3 is a graph showing cathode temperature versus filament voltage for the heaters of FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Heater l, as shown in the drawings, was reverse helically coiled from a 67 mm length of 5.1 mil tungstenrhenium wire. The coiled portion of the heater, that is to say, the body 2, had a length of about 5 mm and legs 3 had a length of about 9 mm. In the case of the prior art heater, shown in FIG. 2, the heater was then merely insulatively coated, usually with an alumina coating 7. All of body 2 and part of legs 3 were coated in order to electrically insulate heater 1 from cathode 4 in which the heater is inserted in operation. The white alumina coating could then be dark coated as shown in US. Pat. No. 3,450,565, to increase its emissivity.

However, in a heater in accordance with this invention, shown in FIG. 1, part of the heater was etched, prior to coating, to reduce wire diameter. All of legs 3 were etched and only enough of body 2 to ensure the etching of all of legs 3, since it is difficult to cut off the etching at the exact juncture of legs 3 and body 2. The heater was etched by immersing legs 3 in 30 percent hydrogen peroxide at a temperature of 85C. for 4 to 7 minutes. This reduced the wire diameter of legs 3 from 5.1 mils, as shown at 5, to 4.2 mils, shown at 6 in FIG. 1.

The heater was then insulatively coated, as usual, With coating 7.

FIG. 3 shows the variation in cathode temperature versus filament voltage for these heaters. The heaters had a rating of 2.1 volts, 800 milliamperes. Curve A represents the prior art heater, that is to say, the unetched heater. Curve B represents the heater of this invention. It can be seen that cathode temperature increases less with increased filament voltage for a heater of this invention, as compared to the prior art unetched heater.

I claim:

1. An insulatively coated wire-wound reverse helix heater comprising a single length of refractory metal wire, said heater having a coiled body portion and two legs, the legs longitudinally protruding substantially parallel to each other from one end of the body, the diameter of the wire in the legs being less than the diam eter of the wire in the body by means of having been etched in a suitable etching liquid. 

1. An insulatively coated wire-wound reverse helix heater comprising a single length of refractory metal wire, said heater having a coiled body portion and two legs, the legs longitudinally protruding substantially parallel to each other from one end of the body, the diameter of the wire in the legs being less than the diameter of the wire in the body by means of having been etched in a suitable etching liquid. 